Using a different methodological approach, Liu et al. performed a cross-sectional case-control study in 215 post-menopausal women with hip fractures and 215 age-matched controls without fractures and reported lower serum irisin levels in women with fractures, while identifying irisin levels in the lowest third and fourth quartiles as being positively associated with a high risk of hip fractures and osteoporosis after adjusting for age, BMI, BMD, FRAX, and physical activity score [52]. Here, FNDC5 is linked to osteoporosis.